From Serendipity to Rational Design: Tuning the Blue Trigonal Bipyramidal Mn³⁺ Chromophore to Violet and Purple through Application of Chemical Pressure

Abstract

We recently reported that an allowed d-d transition of trigonal bipyramidal (TBP) Mn is responsible for the bright blue color in the YInMnO solid solution. The crystal field splitting between a'(d) and e'(d, d) energy levels is very sensitive to the apical Mn-O distance. We therefore applied chemical pressure to compress the apical Mn-O distance in YInMnO, move the allowed d-d transition to higher energy, and thereby tune the color from blue to violet/purple. This was accomplished by substituting smaller cations such as Ti/Zn and Al onto the TBP In/Mn site, which yielded novel violet/purple phases. The general formula is YInMnTiZnAlO (x = 0.005-0.2, y = 0.1-0.4, and z ≤ 0.1), where the color darkens with the increasing amount of Mn. Higher y or small additions of Al provide a more reddish hue to the resulting purple colors. Substituting other rare earth cations for Y has little impact on color. Crystal structure analysis by neutron powder diffraction confirms a shorter apical Mn-O distance compared with that in the blue YInMnO. Magnetic susceptibility measurements verify the 3+ oxidation state for Mn. Diffuse reflection spectra were obtained over the wavelength region 200-2500 nm. All samples show excellent near-infrared reflectance comparable to that of commercial TiO, making them ideal for cool pigment applications such as energy efficient roofs of buildings and cars where reducing solar heat to save energy is desired. In a comparison with commercial purple pigments, such as Co(PO), our pigments are much more thermally stable and chemically inert, and are neither toxic nor carcinogenic.

Exploring the Corundum Structure as a Host for Colored Compounds – Synthesis, Structures, and Optical Studies of (MM′)3TeO6 (M = Mg, Mn, Co, Ni, Zn; M′ = Mg, Mn, Co, Ni, Cu)

“…In the solid state, however, the coordination environments of the transition‐metal ions would be fixed, as determined by the crystal structure. Thus, there is an interest in the exploration of transition‐metal ions in unusual and distorted coordination geometries with a view to the development of new colored inorganic solids, and there have been significant developments in this direction in recent years …”

Robust Yellow-Violet Pigments Tuned by Site-Selective Manganese Chromophores

“…The absorption peak in the short-wavelength region (∼350 nm) is ascribed to the CT of Mn 3+ −O. 9,12,17 In the long-wavelength region (500− 650 nm), the absorption peak is attributed to the symmetryallowed optical transitions (d x 2 −y 2 ,d xy ) → d z 2 of Mn 3+ . 9,12,17 A wide "valley" appears (350−500 nm) between these two absorption peaks, rendering the gray-violet appearance of ZTO-xML.…”

“…For example, Subramanian et al successfully tuned the color of the Mn chromophore via chemical pressure: namely, modulating the apex Mn−O distance in MnO 5 trigonal bipyramid by sizedependent doping in YIn 1−x Mn x O 3 . 9 Both chemical and physical pressure can alter the local environment of a chromophore for a tunable hue hidden under ambient conditions 37−39 and synergistically gauge the pressure-dependent stability of the prescribed pigments.…”

Virtual Issue on Machine-Learning Discoveries in Materials Science

“…A unique blue pigment (YIn 1−x Mn x O 3 ), the only blue pigment discovered in the last two centuries of human history, was also not originally targeted for a potential color pigment application. 27 Applying machine-learning tools can direct research from serendipity toward targeted search via recommendation engines. While being helpful in suggesting materials "outside of the box," giving researchers an opportunity to look for materials within various compositional space, machine-learning approaches also enhance chemical intuition and help to analyze the data in a high-throughput way.…”

“…For example, the original discovery of Nd 2 Fe 14 B was not intended as a study of magnetic properties, and only years later this compound became known as the strongest permanent magnet. A unique blue pigment (YIn 1– x Mn x O 3 ), the only blue pigment discovered in the last two centuries of human history, was also not originally targeted for a potential color pigment application . Applying machine-learning tools can direct research from serendipity toward targeted search via recommendation engines.…”